1. Field of the Invention
The invention relates to a driving method and a structure of a CCD (Charge Coupled Device) area image sensor, and more particularly to a method for driving a CCD area image sensor of an interline transfer type in a non-interlace scanning and to a structure of the same suitable for the driving in the non-interlace scanning.
2. Description of the Related Art
A CCD area image sensor has been developed with a technical progress of semiconductor integrated circuits to the extent that it can be used as a TV camera for commercial-use and home-use. Vigorous studies still continue for improvement of the CCD area image sensor. An improvement of resolution is one of technical problems. While the resolution in horizontal direction can be easily improved by increasing the number of photosensor elements, that in the vertical direction cannot be improved by the same method, because the maximum limit in number of scanning lines is fixed by the TV standard.
In recent years, an attempt for improving the resolution in vertical direction is made by driving the CCD area image sensor in a non-interlace scanning in place of a driving in a conventional interlace scanning. The visual resolution of the non-interlaced picture is theoretically improved by 66 percent, compared to the interlaced picture. The conventional CCD area image sensor of an interline transfer type, however, is designed for the interlace scanning, and cannot be driven in the non-interlacing scanning.
More specifically, the interline transfer type CCD area image sensor has a plurality of lines of photosensors, a plurality of vertical shift registers interposed between the lines of photosensors, a horizontal shift register positioning at ends of the vertical shift registers and an output means producing a voltage signal from quantity of charges transferred through the horizontal shift register. The vertical and horizontal shift registers are made of a series connection of shift stages. One shift stage comprises two sets of storage and barrier electrodes. Charges are stored under one of storage electrodes. Two photo sensors are disposed besides one shift stage of the vertical shift register. At a time, charges accumulated in one of the two photo sensors are transferred to a portion under one of storage electrodes of the corresponding shift stage. The transferred charges are then transferred through the vertical shift register to the horizontal shift register. After the charges are transferred through the horizontal shift register to the output means, the transferred charges are converted into a voltage form signal. In the next time, charges accumulated in the other of the two photo sensors are transferred to another portion under the other storage electrode of the corresponding shift stage and then transferred through the vertical and horizontal shift registers to the output means. This process performs the interlace scanning.
If charges accumulated in the adjacent two photo sensors are simultaneously transferred to the portions under both storage electrodes in one shift stage, the charges are mixed during the transfer through the vertical shift register. The charge mixing is unavoidable, so far as the vertical shift register is driven by usual shift pulses of two or four phases. Due to such charge mixing, the electrical signals corresponding to picture elements can not be obtained with high fidelity. In this way, the non-interlace scanning is impossible by the conventional driving method of the interline type CCD area image sensor.